Spark plug



I March 13, 1934.

H. RABEZZANA S PARK PLUG Filed Feb. 16, 1931 gmentot Patented Mar. 13,1934 UNITED STATES.

SPARK PLUG Hector Rabezzana, Flint, Mich., assignor to A Spark PlugCompany, Flint, Mich., a company of Michigan Application February 16,1931, Serial No. 515,968 4 Claims. (01. 123-169) This invention relatesto ignition devices for firing an explosive mixture in the combustionchamber of an internal combustion engine, and

particularlyto a spark plug of the type wherein u an insulator core ofporcelain or other similar material surrounds a central electrode orwire and is secured between axially spaced portions of an outer metallicshell carrying a grounded electrode or side wire arranged in spacedrelation 19 to the central electrode projecting through the core toafford a gap across which electric current jumps at predetermined timesto fire the combustible charge.

Among other requirements, a spark plug must be leak proof togivesatisfactory and eiiicient service, since the escape of compressed gasesthrough the assembly results in power losses. More importance attachesto this necessity as the degree of engine compression increases and thepresent trend in the industry is toward hotter running and highercompression engine designs. Therefore, the spark plug manufacturer isnow called upon to give more attention than ever before to the matter ofproducing a product that will stand up under these more severeconditions. 7

One of the outstanding difllculties lies in the fact that material mostsuitable for the insulator core has a coefiicient of expansion difierentfrom that of the attachment shell, and therefore, an assembly which maybe leak proof at one temperature may not be so under difierentconditions. Utmost care and precision has had to be exercised in themanufacture and assembly of spark plugs to fit the parts forsatisfactory service throughout the range of temperatures in which theyoperate and so that neither looseness occurs at one temperature nor suchextreme tightness as would impose damaging stresses on the parts andcause fracture thereof at another temperature.

To provide a spark plug which will better meet these new conditions, andin addition eliminate the need for extreme accuracy and therebyfacilitate economical manufacture, is one of the primary objects of thisinvention. To this end there is contemplated, in a specific embodiment,the slitting of the wall of the shell between its core abutment portionsin a manner to weaken the wall and so render it sufllciently resilientto compensate for the differences in expansion while maintaining a tightseal at all times.

A better understanding of the invention will be had from an inspectionof the. accompanying drawing wherein Figure 1 is a side elevation,partly in section, of a spark plug made in accordance herewith. I Figure2 is a vertical section of the outer metallic shell and is taken on line2--2 of Figure 1.

Figure 3 is an elevation showing a slightly modified plug.

Figure 4 is a vertical section of the shell taken on line 4-4 of Figure3, and

Figure 5 is a portion of a shell embodying an alternative arrangement.

Referring to the drawing, the outer shell, which is preferably of steelor other similar material, includes a tubular member 1 having at itsupper end a hexagonal head or tool engaging part 2 and atits lower end askirt 3 which may carry screw threads for engagement within thecustomary spark plug opening of an engine and through which the sidewire or electrode 4, carried thereby, is grounded. The bore of the tu- 7bular member 1 is enlarged at its upper end and provides a taperedshoulder or abutment 5, the outer end of the bore being screw threadedto adjustably receive the gland or nut 6 whose lower end provides atapered shoulder 7 which cooperates with the shoulder 5 in engagingcorresponding shoulders on an enlargement 8 intermediate the ends of theinsulator core 9 which may be formed of porcelain or other suitabledi-electric material, encloses .a central wire or electrode 10 whoselower end is positioned in spaced relation with the grounded electrode4, while its upper end carries screw threads on which is adjustablyengaged a terminal nut 11 for securing thereto the conventional ignitionwiring.

- Since the porcelain core 9 has a relatively lower coefiicient ofexpansion than the steel shell, it is desirable that provision be madeto compensate for this difierence in expansion. As one means ofaccomplishing the purpose the shell may be provided, as shown in thedrawing, with a number of saw kerfs or slots 12-12 extending through theshell wall between the axially spaced abutment shoulders 5 and '7. Thesekerfs are preferably grouped in two sets of two pairs each with theslots of each pair cut inwardly through the wall from diametricallyopposite points and the sets arranged in staggered relation preferablyapart. By the provision of these saw kerfs the wall of. the shellintermediate the abutment portions is sufliciently weakened that thematerial of the wall may give or be distended axially. Upon assembly ofthe parts the gland 6 is drawn up sufficiently to spring the weakenedportion of the shell. As a result a leak proof joint is provided betweenthe shoulder 5 and the portion of the core with which it engages, notonly when the parts are cold but also upon their expansion under heatsince with the relief of stress due to the difference in the rate ofcore and shell expansion the spring portion tends to contract andthereby eliminates looseness. Thus the resilient action of the shellwall serves to automatically compensate for dimensional variations andprevents fracture of the parts, particularly the core, maintainingpractically constant pressure on the sealing gaskets throughout therange of temperature in which a spark plug normally operates.

While the above description refers to a two part metal shell it will bereadily understood that the invention may be applied to the socalledone-piece shell, such as is illustrated in Figures 3 and 4 and wherein,in place of an adjustable gland retainer, the shell 13 is initiallyprovided with an upstanding flange 14 which may be bent down and turnedinwardly over the central core enlargement, as shown in Figure 3 and asillustrated by dotted lines in Figure 4. Figures 3 and 4 are also usedto illustrate a modified arrangement of the wall slitting, there beingshown two slots l515, each in the form of a helix and each extendingthroughout. approximately 270 and being arranged so that adjacent endsof the two slits are in overlapping relation with their intermediateportions on opposite sides of the wall. The function and purpose of thehelical grooves is, of course, the same as the' group of saw kerfs shownin the Figure 1 embodiment.

Instead of providing grouped slits, as heretofore referred to, it willbe obvious that there may be employed a single spiral slit extend ingpreferably throughout more than 380, as is illustrated in Figure 5,wherein 16 indicates -a helical cut to give to the shell portion 1? thedesired elasticity.

Various alternative arrangements of the structure described may bereadily made as come within the scope of appended claims.

I claim:

1'. In a spark plug, an insulator core having an enlargementintermediate its ends to provide, spaced apart shoulders thereon, and ashell surrounding the core and having spaced shoulders corresponding toand engaging with the shoulders of the core to secure the core in theshell and also having in the wall portion thereof between the shouldersa series of horizontal slits grouped in pairs-with the slits of eachpair diametrically arranged and with the pairs angularly related toimpart resiliency thereto, said resilient wall portion being undertension to maintain tight engagement of the core and shell shouldersregardless of relative expansion.

2. In a spark plug, a core having a relatively low rate of expansion anda shell having a relatively high rate of expansion, said shell having apair of spaced seats for engagement with the core and between said seatsa, resilient portion under tension to compensate for the expansionthereof.

. 3..A. spark plug in which the shell has spaced seats to engage thecore, characterized by a resilient shell portion between the seats undertensile strain so as automatically to take up such clearance between theseats and core as might tend to occur due to expansion of said portionunder heat.

4. In a spark plug, an insulator core and a 1 shell having spaced seatsfor engagement with the core to position the parts, said core and 110shell having diflerent rates of expansion under heat, said shell havingcircumferential slits in the region between said seats so constructedand arranged as to impart axial resiliency thereto and said resilientregion being under tensile 115 strain.

HECTOR RABEZZANA.

